Two requirements should be set in providing a highly-efficient gas-exchange apparatus. First of all, a gas-permeable membrane possessing high specific characteristics is required. Secondly, on the basis of this membrane, a design should be provided which has the effect of providing uniform distribution of all the media, effective intermixing, and provides an adequate gas exchange over the entire working surface area of the membrane.
Gas membranes employed in blood oxygenators should have a high permeability with respect to O.sub.2, and still higher permeability relative to CO.sub.2, biological compatibility with blood and sufficient mechanical strength to withstand pressures encountered during operation.
Known in the art are gas-permeable membranes made from polymeric materials: polyethylene, polytetrafluoroethylene (Teflon), polyvinylchloride, natural rubber, dimethylsilicon rubber.
Gas-permeability of membranes made from polymeric materials is a function of gas dissolution properties of a membrane and the diffusion properties of the gas. Therefore, gas-permeability P is a product of solubility S and diffusion coefficient D: EQU P=S.multidot.D
All the above-mentioned membranes of polymeric materials are biologically compatible with blood, but substantially differ from each other in permeability. Thus, the coefficient of permeability of oxygen P.sub.O.sbsb.2 .times.10.sup.-9 for the following substances is equal to:
polyethylene: 0.0002 PA1 polytetrafluoroethylene: 0.0004 PA1 polyvinylchloride: 0.014 PA1 natural rubber: 2.4 PA1 dimethylsilicon rubber: 50.0.
CO.sub.2 -Permeability for dimethylsilicon rubber is about 5 times higher than O.sub.2 -permeability, and for membranes made from other materials this ratio is even higher.
In the first models of blood oxygenators, comprising a housing partitioned by means of a gas-permeable membrane into a blood flow chamber, and gas flow chamber polyethylene films were employed as gas-permeable membranes (cf. J. Close, Nevill "Membrane Oxygenator" in Coll. "Artificial Blood Circulation" ed. By J. Allen, Medgiz Publishing House, 1960, pp. 78-96). Such an oxygenator, to ensure a full-fledged artificial blood circulation, has a gas-exchange surface of 32 m.sup.2 and a capacity of 5.75 l for donor's blood. It has been found that during operation, membranes are covered by depositions and lose the property of non-wettability, thus causing penetration of the liquid into the gas chambers. This limits the time of useful operation of the oxygenator to 2-3 hours.
It has seen from the foregoing that the use of a gas-permeable membrane with a low gas-permeability in blood oxygenators necessitates development of a gas-exchange surface of several dozens of square meters and, consequently, a big (up to 6 liters) volume of donor's blood. Furthermore, such membranes exert a detrimental effect on blood and this limits the time of their use.
For this reason attempts have been made to use, as the material for gas-permeation membranes, dimethylsilicon rubber which has a good gas-permeability and a high biocompatibility. However, pure dimethylsilicon rubber has a low mechanical strength and films with a thickness of only above 100 .mu.m can be obtained therefrom, macrodefects (holes) are frequently formed in such films.
To increase the mechanical strength of the polymer, organosilicon rubber polymers, silicon rubber in particular, application of a nylon fabric was suggested (reinforcing substrate) to obtain reinforced films with a thickness of 125 .mu.m (T. Kolobow, W. Zapol, J. E. Pierce, A. F. Keely, R. L. Replogle and A. Haller "Partial extracorporeal gas exchange in alert new born lambs with a membrane artificial lung perfused via an A-V shunt for periods up to 96 hours", vol. XIV Trans. Amer. Soc. Artif. Int. Organs, 1968, p. 328-334).
The known process for the production of such gas-permeable membranes is to apply silicon rubber onto a reinforcing substrate by casting, followed by rolling of the applied rubber together with the substrate between rolls to create a membrane of uniform-thickness (cf. U.S. Pat. No. 3,325,330 published June 13, 1967). The rubber layer in such membrane fully fills the reinforcing network.
The use of gas-permeable membranes based on silicon rubber has made it possible to produce blood oxygenators for total artificial blood circulation with a working surface of about 6 m.sup.2 and a volume of about 1 liter. However, membranes in these oxygenators have a substantial thickness (125 .mu.m) which is determined by the thickness of the reinforcing screen.
It is known that at a constant permeability coefficient the amount of gas passed through the membrane is inversely proportional to its thickness. Therefore, there is a limitation on the amount of gas passed through the membrane. Furthermore, at the sites of bonding of rubber with the reinforcing screen fibers an insufficient adherence can take place which results in the formation of macroholes and, shutdown of the blood oxygenator.
Most convenient at the present time is the design wherein a plurality of gas-permeable membranes having a central orifice separate alternating blood flow chambers from gas flow chambers. To ensure the blood flow through all of the chambers, the oxygenator is provided with the central inlet and peripheral outlet manifolds; for the gas there are also provided the inlet and outlet gas manifolds. All the membranes have a total working surface sufficient for full artificial blood circulation. Since all currently known gas-permeable membranes employed in blood oxygenators are flexible, to practice such arrangement, it is necessary to use spacing members positioned between the membranes and comprising, as a rule, a rigid grate structures. These members also serve to ensure constancy of the cross-section of the blood flow. However, the use of additional members being in contact with blood exerts a detrimental effect on blood.